1. Field of the Invention
The present invention relates to a navigation system, and specifically to a vehicle navigation system having an anti-theft feature.
2. Description of the Related Art
A vehicle (e.g. automobile) navigation system provides to the driver location information so that the driver can easily understand the vehicle's current location. Generally such a navigation system has the following functions: reading out map data from a map stored on e.g. a CD-ROM or an IC memory card; displaying on a display unit a map image surrounding the location of the vehicle on the basis of the map data; displaying a superimposed location mark of the vehicle on the displayed image; scrolling the map image in accordance with movement of the vehicle; and moving the location mark of the vehicle while fixing the map image on the display.
Some vehicle navigation systems have an anti-theft code input function that prevents normal operation of the system unless a specific identification code is first input. This security measure is in case the system is removed from the car, e.g. by a thief.
FIG. 4 is a block diagram of such a conventional vehicle navigation system. A controller 10 is e.g. a microcomputer, and a GPS (Global Positioning System) receiver 1 detects a present position and speed of the vehicle by means of GPS satellite navigation. A CD-ROM drive 2 ("CD-ROM") has a CD-ROM that stores the map data. There is a display unit 3, and controller 10 controls the display unit 3 to display a map at a present position of the vehicle, a guidance route from a departure point to a destination, and a location mark of the vehicle.
A gyro 4 (gyro-compass) detects a turning angle of the vehicle, and a speed pulse detector 5 detects a speed pulse that is generated in accordance with movement of the vehicle. The controller 10 employs data obtained from the GPS receiver 1, gyro 4, and speed pulse detector 5, and map data from the CD-ROM 2 to detect a present position of the vehicle, its traveling direction, and speed.
A user operation panel 6 operates the navigation system; SRAMS (static random access memories) 7 and 8 store an anti-theft code, and there is a vehicle battery 11.
This system is supplied with electric power from the battery 11. The SRAM 7 loses any stored data when the system is disconnected from the battery 11. A backup battery 9 provides backup power to back up the data stored in the SRAM 8. Since the SRAM 8 is powered by both battery 9 and battery 11, the stored data in the SRAM 8 are maintained even if the system is disconnected from battery 11.
FIG. 5 is a flow chart showing operation of a security function in such a conventional vehicle navigation system. First, when the power switch of the system is switched ON, the controller 10 reads out data stored in the SRAMs 7, 8 at step S1. Next, the controller 10 compares an anti-theft code stored in the SRAM 7 to an anti-theft code stored in the SRAM 8. If the anti-theft code in the SRAM 7 matches the anti-theft code in the SRAM 8, the operation at step S3 brings the security function to a normal end. Thereafter, the system operates normally, namely, detecting a vehicle position based on the GPS signals, searching for a guidance route to a destination, and guiding the vehicle to the destination along the guidance route.
When the vehicle is under maintenance, for instance, the navigation system is disconnected from the battery 11. In such a case, the SRAM 8 is powered by the battery 9, and the SRAM 8 holds its data. However, the anti-theft code stored in the SRAM 7 is lost, and the anti-theft codes read out from the SRAM 7, 8 thereafter will not match at step S2.
In that case, operation moves to step S4 to accept a new anti-theft code input from the operation panel 6. After the new anti-theft code is input, operation moves to step S5 and the controller 10 compares the anti-theft code input from the operation panel 6 to the anti-theft code read out from the SRAM 8. If these anti-theft codes match, operation moves to step S3 to normally terminate the security function.
On the other hand, if the anti-theft code input via the operation panel 6 does not match the anti-theft code read out from the SRAM 8, operation moves to step S6 to normally terminate the security function, and the system halts its operation thereafter.
In this manner, the anti-theft code is stored both in the SRAM 7 powered by the battery 11 and in the SRAM 8 powered by both the battery 11 and battery 9. Accordingly, the two anti-theft codes stored in both SRAMs will not match after the system is removed from the vehicle, because then the anti-theft code stored in the SRAM 7 is lost. By utilizing the non-coincidence of the data stored in both the SRAMs 7, 8, the conventional navigation system detects that it has been disconnected from the battery 11 (e.g., stolen from the vehicle).
However, such a conventional navigation system requires two SRAMs 7, 8 dedicated to this purpose of detecting theft, which increases the cost and size of the system.